This invention relates to the construction of ink drop ejector components of print heads used in ink-jet printing.
An ink-jet printer typically includes one or more cartridges that contain ink. In some designs, the cartridge has discrete reservoirs of more than one color of ink. Each reservoir is connected via a conduit to a print head that is mounted to the body of the cartridge.
The print head is controlled for ejecting minute drops of ink from the print head to a printing medium, such as paper, that is advanced through the printer. The ejection of the drops is controlled so that the drops form recognizable images on the paper.
The ink drops are expelled through orifices that are formed in an orifice plate that covers most of the print head. The orifice plate is typically bonded atop an ink barrier layer of the print head. That barrier layer is shaped to define ink chambers. Each chamber is aligned with, and continuous with, an orifice through which the ink drops are expelled.
The ink drops are expelled from an ink chamber by a heat transducer, which in the past comprised a thin-film resistor. The resistor is carried on an insulated substrate, such as a conventional silicon die upon which has been grown an insulation layer, such as silicon dioxide. The resistor is covered with suitable passivation and cavitation-protection layers, as is known in the art and described, for example, in U.S. Pat. No. 4,719,477, hereby incorporated by reference.
The resistor is selectively driven (heated) with pulses of electrical current. The heat from the resistor is sufficient to form a vapor bubble in an ink chamber, the rapid expansion of which propels a drop through the associated orifice. The chamber is refilled after each drop ejection with ink that flows into the chamber through a channel that connects with the conduit of reservoir ink. The components of the print head, such as the heat transducer, for ejecting drops of ink are oftentimes referred to as drop ejectors. The action of ejecting a drop of ink is sometimes referred to as xe2x80x9cfiringxe2x80x9d the drop ejector.
Early print head designs included a dedicated conductive trace that was interconnected between each resistor and a flexible circuit carried on the body of the cartridge. The flexible circuit mates with a circuit in the carriage that holds the cartridge. Control signals are provided from the printer controller, via the mated circuits, to the individual traces for heating the associated resistors as needed for firing drops.
Print heads having higher resolutions and smaller resistors were introduced as the development of print cartridges continued. While adding more resistors increased the number of ink drops that could be expelled from a print head, it also raised problems with the manufacture of such print heads. In particular, it became impractical and expensive to directly connect individual resistors with dedicated conductive paths to the circuit carried on the cartridge.
To resolve this problem, later versions of print heads were developed with control circuitry (data shifters, buffers, address generators, etc.) placed on the print head substrate with the resistors, instead of in the printer controller firmware. This approach greatly increased the number of resistors that could be addressed (fired) while minimizing the number of interconnections required. U.S. Pat. No. 5,122,812 to Hess, hereby incorporated by reference, describes such a print head having drive circuit components integrated with the resistors on the same substrate. A transistor component of the drive circuit is switched to direct the current pulses through the resistor.
Each drive transistor of conventional print heads is designed so that its resistance (when it is switched on) is low with respect to the firing resistor in order to minimize the power that is used by the transistor and thus made unavailable to the resistor. The transistor area, therefore, is relatively large to ensure this low-resistance requirement is met.
The present invention essentially combines the function of the drive transistor and firing resistor into a single component. In particular, a high-temperature transistor is used as the drop ejector in the print head. The transistor is turned on when an ink drop is required, and the resultant heat generated by the transistor is used for creating the vapor bubble heretofore created by a separate resistor component.
Since, in accordance with the present invention, it is desirable to heat the transistor, the transistor can be made very small to increase its resistance (heat) for a given current and enabling the spacing between transistors (packing density) to be minimized so that high-resolution printing is possible.
In one approach to the present invention, each of the numerous drop ejectors is a silicon carbide transistor. Each transistor is fabricated on a thermally insulating print head substrate or base. Each ink chamber formed in the barrier layer of the print head is located adjacent to one of the transistors so that the heat generated by a transistor in its xe2x80x9conxe2x80x9d state is immediately transferred to the ink in the chamber to generated the vapor bubble that ejects the ink drop.
It will be appreciated that the foregoing use of a transistor as an ink drop ejector (as well as drive control element) greatly simplifies the print head design.
In one preferred embodiment, the components of the print headxe2x80x94including a source, gate, and drain, as well as discrete conductive layers to which the source, gate, and drain are connected for directing power and control signals to the transistorxe2x80x94are covered with layers to protect the transistor for corrosion and cavitation damage.
In another embodiment, the transistor is arranged in what might be considered an inverted orientation so that the ink chamber overlies the silicon carbide substrate of the transistor on a xe2x80x9cpassivexe2x80x9d surface of that substrate that is opposite the surface that carries the primary transistor components (gate, etc). As a result, the silicon carbide substrate of the transistor also serves the purpose of the protective layers so that ink does not reach the transistor components and the deleterious effects of cavitation are avoided.
In yet another embodiment, the silicon carbide transistor substrate is mounted to a print head base that was previously attached to the cartridge. The transistor is thereafter fabricated (patterned, etched, etc.) directly on the attached transistor substrate, thereby avoiding the expense and extra steps involved in separately fabricating the transistors on a print head base (such as a silicon wafer), cutting the wafer into dies and separately attaching each die to a cartridge.
In yet another embodiment, the drop ejector is formed as a resistive diode junction in a silicon carbide substrate. Connections to such drop ejectors are minimized with a row/column multiplexing technique for selectively heating the individual diodes.
Apparatus and methods for carrying out the invention are described in detail. Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.